mysql选择case多列为

Question

I want to set multiple fields with something like this, how can I make it work?

Select TableId,
    CASE
        WHEN StartTime > Appt THEN
            Field AS Field_1
            FieldId AS FieldId_1
        WHEN StartTime BETWEEN Appt AND DATE_ADD(Appt, INTERVAL 1 MONTH) THEN
            Field AS Field_2
            FieldId AS FieldId_2
    END
FROM TableA;

It doesn't work, the syntax is wrong near AS Field_1.

Answer 1

You're trying to perform something similar to macro expansion in a SQL statement, which you cannot do. The simplest way to solve your problem is to write the SQL dynamically in your application code, and execute that statement conditionally from the app.

If you must do it within SQL, you have a couple of options.

Each CASE is a single expression, so you can only produce output fields singly, by having a separate CASE for each column:

SELECT ID,
  CASE WHEN StartTime > Appt THEN Field_A 
       WHEN StartTime BETWEEN Appt AND DATE_ADD(Appt, INTERVAL 1 MONTH) THEN
        Field_B
  END AS field1,
  CASE WHEN StartTime > Appt THEN Field_C 
       WHEN StartTime BETWEEN Appt AND DATE_ADD(Appt, INTERVAL 1 MONTH) THEN
        Field_D
  END AS field2
FROM TableA;

Alternatively, use the statement form of CASE (or IF , which might be simpler) within a stored procedure, to contain multiple versions of your statement and execute them conditionally.

Answer 2

That is utterly broken case syntax. If all you're doing is changing the aliases of those fields, then do that transformation in your client.

eg

SELECT (StartTime > Appt) AS case1,
       (StartTime BETWEEN Appt AND Appt + INTERVAL 1 MONTH) AS case2
       Field,
       FieldID
FROM TableA

if ($row['case1']) then
    $field_1 = $row['Field'];
    $fieldid_1 = $row['FieldID'];
} else if ($row['case2']) {
    etc..
}

Answer 3

You are close, it looks like this

Select TableId,
    CASE WHEN StartTime > Appt THEN Field   ELSE NULL END AS Field_1,
    CASE WHEN StartTime > Appt THEN FieldId ELSE NULL END AS FieldId_1,
    CASE WHEN StartTime BETWEEN Appt AND DATE_ADD(Appt, INTERVAL 1 MONTH) THEN Field ELSE NULL END AS Field_2 ,
    CASE WHEN StartTime BETWEEN Appt AND DATE_ADD(Appt, INTERVAL 1 MONTH) THEN FieldId ELSE NULL END AS FieldId_2

FROM TableA;

Answer 4

I dont know what is field_XXXX content,

But i can make it work with reusing the switch case from the query like this one : (here i am using temporary variable )

SELECT TableId,
    CASE
        WHEN StartTime > Appt THEN
            @temp := 'Field_1' 
        WHEN StartTime BETWEEN Appt AND DATE_ADD(Appt, INTERVAL 1 MONTH) THEN
            @temp := 'Field_2' 
    END AS `Field`,
    CASE
        WHEN @temp := 'Field_1' THEN 'FieldId_1'
        WHEN @temp := 'Field_2' THEN 'FieldId_2'
    END AS `FieldId`
FROM TableA; 

maybe this what you mean.

Answer 5

change the order of your field selection, because case should be first column to be retrieved with alias in mysql the code should be as follows

Select 
    CASE
        WHEN StartTime > Appt THEN
            Field AS Field_1
            FieldId AS FieldId_1
        WHEN StartTime BETWEEN Appt AND DATE_ADD(Appt, INTERVAL 1 MONTH) THEN
            Field AS Field_2
            FieldId AS FieldId_2
    END as <alias>,
TableId
FROM TableA;